Pharmaceuticals are routinely detected at low concentrations in surface waters, but effects on non-target organisms are not well understood. Microcosms were used to assess ecological responses in freshwater ecosystems to a mixture offourtetracyclines commonly used in veterinary and human medicine. Triplicate microcosms were treated with tetracycline, oxytetracycline, doxycycline, and chlortetracycline, resulting in measured time-weighted average total mixture concentrations of 0, 0.080, 0.218, 0.662, and 2.29 microM, respectively. Responses were assessed in terms of structure and function based on measurements of zooplankton and phytoplankton communities, ecosystem productivity, and water quality. Effects were observed for some endpoints > or = the 0.218 microM treatment. The largest responses were concentration-dependent reductions in total phytoplankton abundance and species richness. Phytoplankton abundance recovered to control levels in all microcosms after treatment was terminated, and resilience (time to return to normal operating range during stress) was observed with respectto phytoplankton species richness. Zooplankton were generally unaffected by the tetracyclines. Responses also included decreased water clarity, lower oxygen concentration, and water temperature. Functional endpoints showed varying sensitivity. On the basis of dissolved oxygen concentrations, community respiration (R) increased while primary productivity (P) was unchanged with increased treatment concentration. The effects observed occurred at considerably greater concentrations than are currently measured in the environment, indicating minimal risk to aquatic organisms.